Position transducer having absolute position compensation

ABSTRACT

A scale (110) having apertures representing a binary code (113A-113E) and other apertures (115, 116, 111) generating two compensation signals and moire fringes in cooperation with a mask (105) having tilted apertures (112A) is scanned by a light beam (101A), and the light beam which has passed through the aperture is detected, and an approximate position of the scale is detected by means of the binary code, then a precise position is detected by a signal of the moire fringes which is compensated on the basis of a predetermined algorithm (Table 1) specified by the compensation signals.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

1. Field of the Invention

The present invention relates generally to a position transducer, andmore particularly to an absolute position transducer for detecting anabsolute position with a high accuracy.

2. Description of the Related Art

In position detecting devices using an optical mark reader having aplurality of apertures arranged by a specific rule, a light beam whichhas passed the aperture is converted into an electric signal by aphotoelectric device, and a level of the electric signal is divided intoa plurality of steps, and thereby high precision measurement of positionis realized. In recent development, a plurality of signals which aredifferent from each other in phase are detected with a plurality ofdetectors, and accuracy in measurement has been improved by processingthe plural signals.

In conformity with development of an industrial robot, a higher accuracyin measurement of position is required, and a position transducer of anabsolute type has been introduced to meet the requirement. An example ofa prior art position transducer as shown in the Japanese publishedunexamined patent application Sho No. 59-211822 is illustrated in FIG.11. In the prior art, light beams from a light source 3 are applied on arotary disk 1 through a lens 12. The light beams which has passed aplurality of apertures 7 arranged in a predetermined binary code on therotary disk 1 are detected by respective photoelectric devices 8, 9, 10and 11 which are arranged to the respective apertures of a mask 2 havinga plurality of apertures. A pitch of the apertures on the mask 2 is madeto be to 5/4 of a pitch of the slit 7 on the rotary disk 1. Outputsignals from the respective photoelectric devices 8, 9, 10 and 11 areswitched in turn by switches Sw₁, Sw₂, Sw₃ and Sw₄, and then,fundamental waves of the respective signals are selected by band-passfilter 6. Phases of the respective fundamental waves are compared withthe phases of the outputs from the switches Sw₁, Sw₂, Sw₃ and Sw₄, andthereby an absolute position is detected.

In the above-mentioned detecting method, however, the detected signalsare distorted, and a pure sine wave is obtainable. In order to improvethe disadvantage, deforming the shape of the apertures has been tried,but the disadvantage cannot be completely improved. Furthermore, withrespect to the power source, it is difficult to illuminate uniformly allapertures, and then it is not easy that sensitivity of eachphotoelectric device is equalized.

Additionally, since the output signal of the respective photoelectricdevices are switched, switching noises are superimposed on the signals.Consequently, distortion in the output signal is inevitable due to theabove-mentioned plural problems, and thus non-linear error is producedin position measurement.

On the other hand in the absolute position transducer using apertures ofbinary code, many bits are required to improve precision in measurement,and a lot of photoelectric devices must be arranged, accordingly.Consequently, the position transducer becomes large in size and isexpensive in cost.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a position transducerhaving a higher precision and a less non-linear error than theconventional position transducer.

The position transducer in accordance with the present inventioncomprises:

a movable scale having first apertures representing a position thereofand second apertures arranged parallelly to each other with apredetermined pitch,

a mask located adjacent to the scale and having third aperturescorresponding to the first apertures and fourth apertures for generatingmoire fringes in relation with the second apertures,

a light source for generating a light beam,

an optical means for deflecting said light beam in a manner to scan saidfirst apertures and second apertures,

a photoelectric sensor for detecting light beam which has passed throughthe first apertures and second apertures, and also third apertures andfourth apertures, and

a signal processing circuit for processing output from the photoelectricsensor to produce position data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of a positiontransducer in accordance with the present invention;

FIG. 2 is a front view of a mask of the first embodiment;

FIG. 3(a) and FIG. 3(b) are front views of a scale and the mask inoperating state of the first embodiment;

FIG. 4(a) and FIG. 4(b) are waveforms of output signals of photoelectricdevices in the first embodiment;

FIG. 5 is a block diagram of the circuit for detecting a position in thefirst embodiment;

FIG. 6(a) is a waveform chart of a first compensation signal;

FIG. 6(b) is a waveform chart of a second compensation signal;

FIG. 6(c) is a scale showing measurement ranges of an absolute positionby a binary code;

FIG. 6(d) is a scale showing measurement ranges by moire fringes;

FIG. 7(a) is a waveform of a first compensation signal;

FIG. 7(b) is a waveform of a second compensation signal;

FIG. 7(c) is a diagram of a scale showing measurement ranges of anabsolute position by the binary code;

FIG. 7(d) is a scale showing measurement ranges by the moire fringes;

FIG. 8 is a perspective view of a second embodiment of the positiontransducer in accordance with the present invention;

FIG. 9 is a front view of a mask in the second embodiment;

FIG. 10 is a block diagram of circuit for detecting a position in thesecond embodiment;

FIG. 11 is the perspective view of the position transducer in the priorart.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment]

FIG. 1 shows a configuration of a first embodiment of the positiontransducer in accordance with the present invention. Referring to FIG.1, a light beam 101A emitted from a semiconductor laser device 101 isreflected by a polygon mirror 102 rotating in a direction shown by anarrow and is deflected in a predetermined plane. The deflected lightbeam is collimated by a lens 103. A movable scale 110 and an affixedmask 105 are disposed in the light path of the collimated light beam101B. In FIG. 1, though the mask 105 is illustrated apart from the scale110 so as to help understanding, in the actual position transducer themask 105 is positioned closely adjacent to the scale 110.

The light beam which has passed the scale 110 and the mask 105 isconcentrated on a photoelectric sensor 107 by a lens 106 and isdetected. On the other hand, a photoelectric sensor 108 is faced to thepolygon mirror 102 and detects a reflecting light of the light beam101A, and thereby a synchronizing signal is generated.

The scale 110 has apertures 113A, 113B, 113C, 113D and 113E for togetherforming a 5-bits binary code, apertures 111 for moire fringes, anaperture 115 for a first compensation signal and an aperture 116 for asecond compensation signal, an aperture 114 for a synchronizing signalfor detecting a signal by the moire fringes. The apertures 111 arearranged in parallel with the scanning direction of the light beam 101B.The aperture 114 is a single slit positioned along the length of thescale 110. Gray code, or a kind of the binary code, is optimum to theembodiment.

FIG. 2 is a front view of the mask 105. The mask 105 has three groups ofapertures 112A, 112B, 112C. The apertures 112A are identical with theaperture 111 of the scale 110 in its width and pitch, and are tiltedrelative to the aperture 111 so that a dislocation between an upper endand a lower end of the aperture 112A in a direction of an arrow "d" isequal to three times of pitch of the aperture 111 as shown in FIG. 3(a).In FIG. 2, the apertures 112B pass the light from the apertures 113A,113B, 113C and 114. Moreover, the apertures 112C pass the light from theapertures 113D, 113E, 115 and 116.

FIG. 3(a) is a front view of the scale 110 and mask 105 in theiroperating state. Referring to FIG. 3(a), squares M and N show sectionsof the light beam 101B at the upper position and at the lower positionof the scanning, respectively. The light beam 101 which travelsperpendicular to the sheet of FIG. 3(a) scans vertically from the upperposition M to the lower position N with a predetermined speed. Theparallel lines in light beam 101B of FIG. 1 schematically show beamlight axes at instants of every small time periods. Arrow A shows themoving direction of the scale 110. The aperture 115 is identical withthe aperture 116 in its pitch and width, and location of the aperture115 is phase-shifted by 90° with respect to the aperture 116. The pitchof the apertures 115 and 116 is equal to two times the pitch of theaperture 111, and is made to be equal to two times a minimum detectablelength L of the binary code.

Operation of the position transducer of the first embodiment iselucidated hereafter. Referring to FIG. 1, the light beam 101A emittedfrom the semiconductor laser device 101 is deflected by the rotarypolygon mirror 102. Subsequently, the light beam is collimated by thelens 103, and scans the scale 110 from the upper edge to the lower edge.The light beam which has passed the apertures of the scale 110 and themask 105 is concentrated on the photoelectric sensor 107 by the lens 106so as to be detected thereby. Thus, a signal of binary code is generatedon the basis of the light beam which passed the apertures 113A, 113B,113C, 113D and 113E. The signal of binary code is processed by awell-known circuit, and an absolute position of the scale 110 isdetected with an accuracy corresponding to the pitch of the aperture ofLSB (least significant bit) of the binary code. Additionally, as shownin FIG. 3(a), moire fringes are formed by the apertures 111 of the scale110 and the apertures 112A of the mask 105, and dark portions D₁, D₂ andD₃ and bright portions B₁ and B₂ are alternately generated. The moirefringes move in the direction perpendicular to the moving direction ofthe scale 110, as shown by an arrow C.

FIG. 3(b) is a front view of the scale 110 and the mask 105 wherein thescale 110 is shifted to a direction of an arrow A₁ by a distance d₂. Thedark portions D₁, D₂ and D₃ and the bright portions B₁ and B₂ moveupward by a distance d₁. The dark portions D₁, D₂ and D₃ and the brightportions B₁, B₂ and B₃ are detected by the photoelectric sensor 107(FIG. 1) by scanning the moire fringes with the light beam 101B.

FIG. 4(a) shows a waveform of an output signal of the photoelectricsensor 107, and FIG. 4(b) shows a waveform of an output signal of thephotoelectric sensor 108. A signal 108A of the photoelectric sensor 108is firstly output by scanning with the light beam 101A. Subsequently,the output signal of the photoelectric sensor 107 is output as shown inFIG. 4(a). Referring to FIG. 4(a), pulse-shaped signals 113M and 113Lare of the apertures 113A, 113B, 113C, 113D and 113E of the binary code.A moire synchronizing signal 114A and compensation signals 115A and 116Aare generated by the apertures 114, 115 and 116, respectively. Signals111A and 111B are generated by the moire fringes. The signal 111A is anoutput signal by the moire fringes which are present at a moirereference position MP as described hereafter, and the signal 111B is asignal output by the moire fringes moving from the reference position.In FIG. 4(a), a time "Tc" is a cyclic time of the moire fringes, and atime "To" is a time period between the moire synchronizing signal 114Aand the moire reference position MP. A time "T" is a time period betweenthe moire synchronizing signal 114A and an intersection of the outputsignal 111B and a base level of the moire signal. The moire referenceposition MP is a constant position determined by measurement afterassembly of the position transducer.

FIG. 5 is circuitry of a signal processing circuit in the firstembodiment. The output signals of the photoelectric sensors 107 and 108are inputted into a binary code selecting circuit 130, and a binary codesignal is selected. Then the binary code signal is inputted into binarycode converter 131, in which the binary code is converted into a data ofan absolute position of the scale 110, and the absolute position ismemorized in a memory 132. On the other hand, the moire signal 111B isselected by a moire signal selecting circuit 111C, and is inputted intoa calculates circuit 111D which calculating the time "T" and acalculating circuit 111F for calculating the time "Tc". Moreover, themoire synchronizing signal 114A is selected by a moire synchronizingsignal selecting circuit 114B, and is inputted into the calculatingcircuit 111D. In the calculating circuits 111D and 111F, the times "T"and "Tc" are calculated on the basis of the clock signal from a clockoscillator 121. The data of the times "T" and "Tc" are inputted into thecalculation circuit 111G with data of the time "To" measured in a memory111H, and a value "F" is obtained by calculation of the followingequation (1):

    F=(T-To)/Tc                                                (1).

The value "F" represents a ratio of a shift distance of the moirefringes to one pitch of the moire fringes; in other words, it representsa shift ratio of the moire fringes which is detected by travel of themoire fringes from the moire reference position.

A product of the shift ratio and the minimum detectable length L iscalculated by a calculation circuit 133. The value of the productrepresents an infinitesimal distance in the minimum detectable length L.Subsequently, the infinitesimal distance is added to the positiondetected by the binary code, and thereby, an accurate absolute positionis detected.

In calculation of the equation (1), since a dimension of a time (T-To)is divided by a dimension of a time (Tc), the quotient representing theshift ratio is an absolute number. Therefore, the resultantinfinitesimal distance is not influenced by fluctuation of the periodictime of the clock signal or the moire fringes which is caused bydislocation in arrangement of the scale 110 and the mask 105.

Moreover, since the trigger signal 114A which is a reference signal inmeasurement of the time "T" and "To" is generated by the aperture 114which is located adjacent to the apertures 111, the time "T" and "To" isnot influenced by dislocation in assembly of the scale 110 or byfluctuation of the scanning speed of the light beam. Consequently, ahigher accuracy in measurement of the infinitesimal distance isrealizable.

FIG. 6(a) shows a waveform of a first compensation signal, and FIG. 6(b)shows a waveform of a second compensation signal. FIG. 6(c) shows ascale showing measuring ranges of the absolute position by means of thebinary code, and in the embodiment, the minimum detectable length L is10 μm. The numerals on the scale, for example +10 μm, -20 μm or thelike, represent a shift distance of the scale 110 from an initialposition z. FIG. 6(d) shows a scale representing measuring ranges bymeans of the moire fringes.

In case that respective edges KL of the signal in FIG. 6(b) coincidewith boundaries KM of the measuring range of distance by moire fringesas shown in FIG. 6(c), an accurate absolute position is obtainable byadding a distance by moire fringes to an absolute distance detected bythe binary code.

In the actual position transducer, however, since a diameter the lightbeam 101B is larger than two times of the pitch P₂ of the aperture 115,an output signal of the photoelectric sensor 107 is a triangular wave.Then, a duty ratio of the first compensation signal is varied byvariation of condition of the circuit for reforming the waveform of thetriangular wave, and thus, the edges KL cannot coincide with theresponding boundaries KM as seen from FIG. 7(b) and FIG. 7(d).

In the following, the reason that the error is produced in therepresented value is elucidated.

Referring to FIG. 7(c) and FIG. 7(d), in case that a position "a" isdetected, for example, which is +5.5 μm in truth value from a position"z", a distance of -4.5 μm should be detected in the range of +10 μm ofthe scale of FIG. 7(c) as a measured value by moire fringes. Thus, thedistance between the positions "a" and "z" is expected to be +5.5 μm(given by +10 μm-4.5 μm=+5.5 μm). As a matter of fact, however, since anedge KL₁ of the signal of FIG. 7(b) is dislocated rightward, the rangeof zero μm is detected by binary code. Therefore, a totalized value ofthe absolute position detected by the binary code and the infinitesimaldistance by the moire fringes becomes -4.5 μm (given by 0 μm-4.5 μm=-4.5μm). Consequently, an error of 10 μm is present.

In order to prevent the error as described above, in the presentinvention, in case that a measured value "F" by the moire fringes iswithin the range defined by relation (2), a detected absolute positionby the binary code is compensated as follows by using a signal of thefirst compensation aperture 115 as shown in FIG. 7(a) and a signal ofthe second compensation aperture 116 as shown in FIG. 7(b).

    +2.5<F, F<-2.5                                             (2).

With respect to the position "a" in FIG. 7(d), for example, since themeasured value "F" of moire fringe is -4.5 μm, the absolute position ofbinary code is compensated in the following manner:

In case that the second compensation signal of FIG. 7(b) is "0" and thefirst compensation signal of FIG. 7(a) is "1", a value of 10 μm is addedto a detected value of the absolute position. Consequently, the trueabsolute position is represented by calculation of the relation (3).

    10 μm+(-4.5 μm)=+5.5 μm                           (3).

The algorithm of the compensation is shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Measured Level of first                                                                           Level of second                                                                           Compensation                                  value F of                                                                             compensation                                                                             compensation                                                                              or non-                                       moire fringe                                                                           signal     signal      compensation                                  ______________________________________                                         ##STR1##                                                                              0          0 1         -L (-10 μm)                                                                non-compensation                                       1          0           non-compensation                                                  1           -L                                                                            (-10 μm)                                    ##STR2##                                                                              0          0 1         non-compensation +L                                                           (+10 μm)                                            1          0           +L                                                                            (+10 μm)                                                       1           non-compensation                              ______________________________________                                    

With respect to the position "b" for example, compensation of theabsolute position "b" is calculated in accordance with the Table 1 asdescribed hereinafter: since the measured value "F" is larger than +2.5μm (F>+2.5 μm), the first compensation signal is "1" and the secondcompensation signal is "1", then a true absolute position is obtained bysubtracting 10 μm from the sum of an absolute position of binary codeand a measured value of moire fringes. The calculation is as follows:

    (-10 μm)+(4 μm)-10 μm=-16 μm                   (4).

Consequently, the true absolute position of -16 μm can be detected.

Moreover, with respect to the position "d", since the measured value "F"of moire fringes is smaller than -2.5 μm, the second compensation signalis "1" and the first compensation signal is "0", a value of 10 μm isadded to the detected absolute value. Consequently,

    10 μm-4.5 μm+10 μm=+15.5 μm                    (5).

The above-mentioned calculations are accomplished in a compensationcircuit 145 having a microprocessor and memories.

[Second Embodiment]

FIG. 8 is a perspective view of a second embodiment of the positiontransducer in accordance with the present invention. Referring to FIG.8, the optical system which is composed of the semiconductor laserdevice 101, the polygon mirror 102, the lenses 103 and 106 and thephotoelectric sensor 107 is identical with that of the first embodiment.In the second embodiment, a trigger aperture 231 is located at theuppermost portion of a scale 223. A wedge-shaped aperture 230 isdisposed under the trigger aperture 231 on the scale 223. Moreover,apertures 232 and 233 for compensation signals, a trigger aperture 234for generating a trigger signal for the moire fringes signal areprovided under the wedge-shaped aperture 230 in the cited order. On thelowest portion of the scale 223, apertures 228 for producing the moirefringes are also provided.

FIG. 9 is a front view of a mask 224. Referring to FIG. 9, the lightbeam 101B passes through the apertures 231A, 230A, 232A, 233A, 234A and229 after passing through the apertures 231, 230, 232, 233, 234 and 228,respectively. In the second embodiment, an absolute position of thescale 223 corresponds to a width W of the wedge-shaped aperture 230, andthe absolute position is detected by measuring a time period of thelight beam 101B crossing the wedge-shaped aperture 230. The outputsignal of the photoelectric device 107 detecting the light beam 101Bcrossing the wedge-shaped aperture 230 is converted into a digitalsignal by utilizing an output signal by the aperture 233 as A/Dconverting signal, and the absolute position is detected with anaccuracy which is determined by a pitch of the apertures 233. In thesecond embodiment, a signal which serves the same function as the signal108A in the first embodiment as shown in FIG. 4(b) is obtained by meanof the apertures 231. Therefore, the photoelectric device 108 in thefirst embodiment is deleted. The above-mentioned method is alsoapplicable to the first embodiment.

The moire fringes which are formed by the apertures 228 and 229 aredetected by the photoelectric sensor 107, and in a similar manner of thefirst embodiment, an infinitesimal distance is detected. Then theinfinitesimal distance is added to the absolute position, and thecompensation process is accomplished on the basis of the Table 1 asshown in the first embodiment.

FIG. 10 is a block diagram of the signal processing circuit of thesecond embodiment. Referring to FIG. 10, a signal selecting circuit 240discriminates signals of the wedge-shaped aperture 230 and triggeraperture 233 from the output of the photoelectric sensor 107, and theoutput of the signal selecting circuit 240 is inputted to a positiondetecting circuit 241 for detecting an absolute position. The absoluteposition calculated by the position detecting circuit 241 is memorizedin a memory 242. Other process and circuit therefor is identical withthat of the first embodiment.

Though the optical-type binary code and moire fringes are used in theforegoing embodiments for detecting the position, another positiondetecting method, such as, using magnetic recording process is alsoapplicable to the present invention. The light source and the opticalsystem in these embodiment are not restricted by the above-describedmethod, and a galvano-mirror is usable instead of the polygon mirror,for example.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A position transducer comprising:a first positiondetecting means for detecting an absolute position with a predeterminedminimum detectable length, second position detecting means for detectinga position within a range of said minimum detectable length, firstbinary-value signal generating means for generating an alternatingbinary-value signal having a wave length of two times said minimumdetectable length, second binary-value signal generating means forgenerating an alternating binary-value signal having a wave length oftwo times said minimum detectable length and a phase difference of 90degrees with respect to said alternating binary-value signal of saidfirst binary-value signal generating means, and compensation means forcompensating said absolute position by using said position detected bysaid second position detecting means, and said alternating binary-valuesignals of said first binary-value signal generating means and saidsecond binary-value signal generating means.
 2. A position transducer inaccordance with claim 1, whereinsaid first position detecting meanscomprises a binary code, and said alternating binary-value signals areformed by a signal of the least significant bit of said binary code. 3.A position transducer in accordance with claim 1, whereinsaid secondbinary-value generating means generates said alternating binary-valuesignal by phase-shifting the signal of the least significant bit of saidbinary code by substantially 90 degrees.